I think I've put my finger on what makes the kind of AU I like to read and write versus the kind I don't. To whit, I don't like the kind that keep the same events while changing the circumstances (eg, the majority of high school AUs). That's just a retread, and while the trappings of the events change, neither the events themselves nor the characters that arise from them do.

I find that boring.

I much prefer the kind of AU that changes the canon events to see how that will make the characters different. While it's possible to write the changed-setting type of AU and still do good characterization, it is not a form that encourages any such thing; far too many fan-authors wind up writing very shallow characterization when they write those AUs. An AU that changes the events, whether or not the setting changes, demands that the author put more work into defining just what they identify as a character's core traits. Not everyone does the work, and when they don't it's a hot mess, but the form encourages it a lot more. There's less leeway, in this type of AU, to let familiar plot stand in for actual characterization.

It's a basic plot-driven versus character-driven divide, I think. I will always be on the character-driven side, and I find most plot-driven writing boring and shallow. (No doubt, plot-driven writers/readers find character-driven writing far too meandering.) And, above and beyond that, I've already seen the canon plot once; I really don't need to see it again. It's more interesting to do something the canon didn't do.